Roof Cleaning and Lichen Assessment

Algae is the dark streak you see on many roofs. It is a thin stain, and the shingle industry group ARMA names one kind, Gloeocapsa magma, as the most common cause. Moss is thick, green and fuzzy. Mold and mildew show up as dark spots that look a lot like algae. Lichen is the crusty, scaly patch that holds on tight.

They need the same things to grow: damp and shade. They do not do the same things to your shingles. That is why we start by finding out which one you have.

Clumps of mold, mildew and moss sit on top of the shingle. A soft wash kills them and rinses them away. The roof comes back to its true color. A gentle wash does not pull granules off the way scrubbing does.

Moss is the one to watch. It acts like a sponge and holds water against the roof. The Oregon State University Extension says to trim trees so the sun reaches the roof, because cleaning alone will not stop it from coming back.

This is the part no one likes to say. Most of the time, when lichen comes off, it has already eaten the granules under it. We are not washing the granules off. The lichen already did that, over years. All that is left is the tar and the fiberglass.

That is why we look before we wash. We take photos before we start. We check the granules and the shingle underneath, and we talk with you about what we see. If the roof can take it, we clean it. If cleaning would show bare patches, we tell you, and we talk about a repair or a replacement instead. No pressure to buy either one.

Pro roof washers use water mixed with sodium hypochlorite, which is pool-strength bleach. A soap-like additive called a surfactant helps it cling to the shingles instead of running off. Many pros mix the bleach down to about 3 to 6 percent on the roof. That strength is common practice, not a shingle maker rule. ARMA gives no number.

Mold, algae and light moss do well at the lower end. Heavy moss and lichen can call for a stronger mix or a second pass. A pump moves it at about garden-hose pressure. We do not use a pressure washer, an acid or a stiff brush.

Before we spray, we wet the plants. After, we rinse them and neutralize the runoff. We follow your shingle maker first. Where the maker says nothing, we use what careful pros use, and we tell you what that is.

A clean roof will not stay clean forever. ARMA says cleaning results can be temporary. How long it lasts depends on shade, trees and damp.

Three things help. Trim trees so the sun and wind reach the roof. Keep needles and leaves out of the valleys. Ask about zinc or copper strips under the ridge, which let rain carry metal down the slope and slow regrowth. Strips protect only part of the slope below them, and zinc can stain or damage copper gutters, so we check that first. When you do replace the roof, algae-resistant shingles have copper in the granules. Makers back them with warranties of about 10 to 30 years, depending on the shingle.